Electrolytic preparation of chlorine pentafluoride



March 12, 1968 E. A. LAWTON ETAL 3,373,096

ELECTROLYTIC PREPARATION OF CHLORINE PENTAFLUORIDE 2 Sheets-Shem 1 FiledSept. 24, 1964 INVENTORS EMIL A. LAWTON HOWARD H. ROGERS FIG-- 2.

United States Patent O 3,373,096 ELECTROLYTIC PREPARATION OF CHLORINEPENTAFLUORIDE Emil A. Lawton and Howard H. Rogers, Woodland Hills,

Calif., assignors to North American Rockwell Corporation, a corporationof Delaware Filed Sept. 24, 1964, Ser. No. 399,110 3 Claims. (Cl.204-59) This invention relates to an electrolytic process for prepartionof chlorine pentafluoride, ClF from chlorine and hydrogen fluoride.

A method for the preparation of C11 by electrolysis of a solution ofchlorine trifluoride and hydrogen fluoride is described in patentapplication Ser. No. 294,765, filed July 12, 1963, now Patent No.3,303,401. As therein mentioned, C11 is an extremely high-energyoxidizer of greater oxidizing potential than chlorine trifluoride whichfinds utility as an oxidizer for rocket propellant fuels. The boilingpoint of C11 is about 14 C.

Compared with chlorine, chlorine trifluoride is expensive. Hydrogenfluoride is relatively inexpensive. Therefore, the use of the relativelyinexpensive raw materials, chlorine and hydrogen fluoride, for synthesisof ClF as provided by this invention, offers a significant advantage.

Broadly stated, the invention comprises subjecting chlorine and hydrogenfluoride with a conductivity additive, to an electric current in anelectrolytic cell, and collecting the chlorine pentafluoride which isevolved from the cell. The chemical changes which occur at the anode ofthe cell may be represented by the following equations:

In addition to the chlorine pentafluoride, other resultants of theelectrolysis are ClF, HCl, and possibly F and when impure reactants areemployed various contaminants are produced, such as C CIO F, and ClO F.The chlorine pentafluoride may be separated from chlo rine and from theother resultants and contaminants by conventional procedures includinglow temperature distillation.

With respect to the conductivity additive, any of the alkali metalhalides are usable, preferably an alkali metal fluoride. Pure hydrogenfluoride being practically nondissociated, the conductivity additivefurnishes the ions by which the electric current is carried through theelectrolytic cell; however, the additive is not consumed in the overallprocess, the additive being continually regenerated by the fluorine ofthe hydrogen fluoride. In cases Where the additive is a halide otherthan fluorine, mere substitution of the non-fluorine halogen by fluorineatoms from the hydrogen fluoride occurs with evolution of thecorresponding hydrogen halide.

The invention is hereinafter illustrated by descriptio with reference tothe accompanying drawing, in which:

FIG. 1 is a diagrammatic representation of a suitable laboratoryapparatus for the electrochemical synthesis of chlorine pentafluorideaccording to the process of this invention;

FIG. 2 is a detail section on an enlarge-d scale, through theelectrolytic cell of the apparatus, taken upon a plane which isindicated on FIG. 1 by line 22;

FIGS. 3, 4, and 5 are graphs in which anode potential is plotted againsttime of operation of the cell for showing the behavior of the potentialat the anode of the cell.

In FIG. 1 of the drawing, reference numeral 10 designates anelectrolytic cell of stainless steel comprising a double-walled tank 12having a circumferentially conlinuous flange 13 at its top, a cover 15secured to the flange with a gasket 16 of Teflon between the cover and"ice the flange. The tank has a drain 18 for emptying the cell and hasan inlet 19 and an outlet 29 for continuous flow of a coolant throughthe space between the tank walls. Spaced apart within the tank by aboutone-half inch are two plate electrodes, anode 22 and cathode 23 (50 sq.cm. on each face) of a metal, e.g., nickel, which is not easily solublein the reactants and does not form an insulating anodic film. Theelectrodes are suspended from the cover 15 by posts 25 electricallyconnected by leads 27 to a power source at 29, e.g., a continuouslyvariable, full-wave rectified system including calibrated meters formeasurement of current and voltage.

An inert gas, e.g., helium, is preferably employed for purging theapparatus and to serve as a carrier for the product. There is a valvecontrolled line 32 extending through the cover of cell 10 and adapted tobe connected at its outer end to a cylinder (not shown) of helium underpressure. A gauge 33 connected to the line 32 indicates the pressure inthe cell. For supplying the reactant chlorine, a flow line 35,containing a valve 36, a flow meter 37, and a metering valve 37',extends through the cell cover 15 and is adapted at its outer end forconnection to a cylinder (not shown) of chlorine. Within the cell, thechlorine inlet line 35 continues as a section 33 of Teflon having a legportion 39 paralleling the lower edge of the anode plate 22. The leg 39is closed at its end 40 and is provided with pinholes 41 for bubblingchlorine into the cell. The anode is preferably bent as along bend line42 (FIG. 2), and the section 33 is correspondingly bent, whereby thebent portion of the anode plate 22 extends over the perforated legportion 39 so that bubbles of chlorine will impinge against the anodeplate instead of rising unobstructed in the cell.

For supplying hydrogen fluoride to the cell, there is a valve-controlledinlet tube 44 adapted to be connected at its upstream end to a supply ofliquid hydrogen fluoride and at its downstream end to the inside of theelectrolytic call. A branch 45 on the line 44 serves to admit a solutionof the conductivity additive in hydrogen fluoride.

Standing upright from the center of the cell 10 is a condenser 47 withits lower end extending through the cell cover. The upper end of thecondenser is connected by a tube 48, controlled by a valve 4-9, to oneend of an absorber column 50 filled with sodium fluoride pellets forabsorbing any hydrogen fluoride gas which may be carried over from thecondenser 47. A bypass 51 controlled by a valve 52 is connected to thetube 48 upstream of the valve 49 for preliminary removal of gases. Atrain of U-tube traps is connected to the downstream end of the HFabsorber 50, such train comprising a flow line 53, first and secondU-tube traps 54 and 55 of PEP-Teflon, and a flow line 56 leading to aplace for storage at 57 for the chlorine pentafluoride. A gauge 59connected to the line 56 indicates the pressure in the train of traps. Apurge outlet 60 is connected adjacent the downstream end of line 56. Thevalves of the apparatus are formed of Monel metal and except whereotherwise explained above, the rest of the apparatus is formed ofstainless steel.

In operation, the apparatus is preferably first flushed by flowinghelium from the inlet 32 to the purge outlet 60. Hydrogen fluoride andthe conductivity additive, e.g., sodium fluoride, are then introducedinto the cell to a level covering the plate portions of the electrodes.It is preferred to purify the hydrogen fluoride, if not purified whenintroduced and for that purpose electric current is passed between theelectrodes 22 and 23, the valve 49 is closed and the bypass 51 is openedthereby to remove contaminants, e.g., sulfur and oxygen compounds fromthe hydrogen fluoride, with the helium or other inert gas from line 32as a carrier flowing over the surface of the liquid in the cell, thenceup through the condenser 47 and out through the bypass 51. Thereafter,valve 49 is opened,

3 valve 52 is closed, branch line 57 leading to storage is closed, purgeoutlet 60 is opened, and chlorine is bubbled into the cell through theperforations of its inlet tube section 39 preferably at a rate of fromabout 60 to 240 cc. per min. The electric power source is thenenergized, and the gaseous products (including ClF pass into thecondenser 47. The condenser is cooled as with methanol at from about--l0 C. to C. whereby most of the hydrogen fluoride vapors are refluxedback into the cell. The carrier gas, chlorine and the chlorinepentafluoride along with the other products of the electrolysisreactions, 1

i.e., F and ClF, then flow through the HF absorber 50, and thencethrough the train of cold traps 54-55. The first cold trap 54 ispreferably cooled to 78 C. by envelopment in a hath (not shown) of DryIce and trichloroethylene to collect chlorine and most of the ClF Thesecond cold trap 55 is preferably cooled to -l96 C. by a bath of liquidnitrogen for collecting ClF, ClF and the remainder of the chlorine.Thereafter, a purge outlet 66 is closed, the train of traps is closedoff from the condenser and the cooling baths are removed whereby thecollected products in the traps pass as gases to storage 57.

The following table sets forth particulars of operating conditions forseveral examples of the practice of the process of this invention usingthe apparatus described above:

cell when helium instead of chlorine wasbubbled from the tube section39, and thereafter when C1 was substituted for the helium. No potentialrise occurred and no ClF was produced until chlorine was bubbled intothe cell. The degree of voltage rise is, of course, a function of thecell geometry and temperature of electrolyte.

Each of the tests 3, 6, and 7 in the hereinabove table were conducted at0.5 amp on a day following an overnight rest of the cell which hadtheretofore been operated at 1.0 amp. The graph of FIG. 5 showsproduction of ClF under conditions of operation Where the cell hadpreviously experienced a rise in anode potential.

Though chlorine was bubbled into the cell at different rates of 60, 120,and 240 cc./min. no significant difference in yield of ClF occurred. Solong as chlorine is present, ClF will be produced. To avoid highconcentrations of chlorine appearing in the collection traps 54 and 55,the rate of chlorine additive should be controlled to reduce the extentof chlorine bubbles at the surface of the liquid in the cell. Whenliquid chlorine was added to the cell at about 38 C., ClF was producedbut in reduced yields.

Another variable to be considered as having an effect on the process ofthis invention is that of the pressure in the electrolytic cell. For theexamples in the above table, pressures of about 1 atmosphere wereemployed. Lower Test Current Voltage Time in Temp. Percent The termpercent yield, which appears in the above table, is calculated as 100times the quotient of the actual yield of ClF in grams divided by thetheoretical yield, with reference to Formula No. l hereinabove. Thetheoretical yield in grams equals the number of coulombs passedmultiplied by the gram equivalent weight of ClF i.e., 130.5 divided by482,500 coulombs.

In the examples of the above table, the molar ratio of hydrogen fluorideto sodium fluoride as the alkali metal halide was initially about 40to 1. If such ratio is increased to about 80 to 1, for example, theprocess of this invention will proceed but a substantial increase involtage Will be required to attain the same amount of electric current,whereby the efl'iciency of the process will be substantially reducedfrom a practical standpoint. Decreasing such ratio decreases the voltagerequired to attain the same current. Saturated solutions of the alkalimetal halide in the hydrogen fluoride may be employed.

The amount of current employed aifects the efliciency of operation ofthe process of this invention. Generally, the efliciency of the processincreases with an increase of current density (amps per unit area ofeffective electrode surface). With the above described laboratoryapparatus, currents of one-half to two amps were employed; h-owever, thegraphs of FIGS. 3, 4 and 5 show that the current density is not assignificant a factor to the synthesis of ClF as is the incidence of apotential rise at the anode when operating at a constant current.Referring to FIG. 3, it shows that when the illustrated cell wasoperated at a constant current of 0.5 amp (6 ma/cm?) for a period of 60min, no ClF was produced, and the anode potential, as measured by amercury-mercurous fluoride reference electrode, remained fairly constantat slightly above 4 vol-ts. Thereafter, the current Was increased to adensity of 12 rna./cm. whereupon the anode potential increased steadilythroughout :a period of about minutes to slightly above 6 volts and GEwas noted as being pressures lower the boiling point of HF and therebyincrease the amount of HF carryover. An increase in pressure raises theboiling point of HF and increases the concentration of chlorine in theelectrolyte and therefore permits operation at higher temperatures. 7

Turning now to the factor of temperature as an operating condition forthe process of this invention, for the laboratory apparatus illustratedin the drawing and described above, a range of temperature of from 0 tol4 C.. may be set as a limited range, at atmosphericpressure. Operationat below l4 C. would result in a batch process instead of theadvantageous continuous process by requiring periodic distillation forrecovery'of ClF Increasing the solution temperature in the cell to above0 C. produces too much volatilization of HF for convenient laboratoryhandling. It is important to note however, that for an electrolyticprocess, an increase in temperature is advantageous because it increasesthe conductivity of the electrolyte whereby less voltage and hence lesspower is required for the same current density. A preferred temperatureis about l0 C.

Contaminants which contain oxygen and through combination with fluorineproduce undesirable products such as C10 and C10 should'be avoided.Sulfur would terfere with the efliciency of the reaction as it is moreeasily fluorinated than chlorine. Silver fluoride and thalium fluoride,which are soluble in hydrogen fluoride, would lend conductivity to thesolution in the cell; however,.it is expected that silver would plateout at the cathode.

In the above described laboratory preparation of C11 5 according to theprocess of this invention, the GR in storage 57 may be separated fromthe other components which were evolved from the cold traps 5'4 and inany conventional separation operation, e.g. passing the contents ofstorage 57 through a low temperature fractional distillation column torecover the ClF It will be understood that it is intended to cover allchanges and modifications of the examples of the invention herein chosenfor the purpose of illustration which do not constitute departures fromthe spirit and scope of the invention.

Having described the invention, what is claimed is:

1. A process for synthesizing chlorine pentafluoride comprising thesteps of passing an electric current between spaced electrodes in anelectrolyte comprising chlorine, hydrogen fluoride, and an alkali metalhalide, whereby chlorine pentafluoride is evolved, and collecting theevolved chlorine pentafluoride.

2. The process of claim 1 in which said alkali metal halide is afluoride.

3. The process of claim 1 in which a laboratory apparatus is employedcomprising an electrolytic cell having two plate electrodes spaced apartby a distance of about one-half inch, each electrode being of about 50sq. cm. per plate face, the apparatus being operated under the 01-lowing conditions: the molar ratio of HFzalkali metal halide is fromabout 80:1 to saturation of the alkali metal halide in HF; the pressurein the cell is about atmospheric; the temperature of the solution in thecell is from about 0 to about 14 C.; and the current is initiallymaintained References Cited UNITED STATES PATENTS 5/1967 Yodis 20459 10/1966 Donohue et al. 204-101 HOWARD S. WILLIAMS, Primary Examiner.

1. A PROCESS FOR SYNTHESIZING CHLORINE PENTAFLUORIDE COMPRISING THESTEPS OF PASSING AN ELECTRIC CURRENT BETWEEN SPACED ELECTRODES IN ANELECTROLYTE COMPRISING CHLORINE, HYDROGEN FLUORIDE, AND AN ALKALI METALHALIDE, WHEREBY CHLORINE PENTAFLUORIDE IS EVOLVED, AND COLLECTING THEEVOLVED CHLORINE PENTAFLUORIDE.